90 STUDIES IN GENERAL PHYSIOLOGY 



light rays determines the orientation of the organs of the 

 plant. It is characteristic of the organs of sessile plants that 

 heliotropic curvatures are produced when the plant is illu- 

 minated from but one side. A growing stem continues to 

 bend when illuminated from one side only until the growing 

 tip lies in the direction of the rays of light. Progressive 

 movement in the direction of the rays of light, which is the 

 rule for free-moving animals and plants, is of course impos- 

 sible for sessile organisms. Everyone who has cultivated 

 flowers in a room has no doubt observed the heliotropic 

 bendings in the plants. The question now arises whether 

 these heliotropic curvatures can also be produced in sessile 

 animals when illuminated from one side only. I shall show 

 in the following pages that this is, indeed, the case. 



1. The experiments described here were made on the large 

 marine Annelid, Spirographis Spallanzanii. It lives in a tube 

 which is quite flexible, yet sufficiently rigid to keep the ani- 

 mal in a definite position. The tube is formed from the 

 secretions of the animal. The aboral end of the tube is 

 fastened (by a secretion) to stones or other solid objects. 

 The gills of the animal, which are arranged at its anterior 

 end in several spiral turns radial to the longitudinal axis of 

 the animal, are usually found unfolded and projecting beyond 

 the open end of the tube. As the tube is almost impervious 

 to light, the latter will act chiefly upon the gills. So far as 

 we know at present, the animal has no eyes. 



The animal can move freely inside the tube, the inner 

 surface of which is perfectly smooth, and can be removed 

 from it without the slightest injury by cutting open the tube. 

 I have occasionally seen the worm leave the tube of its own 

 accord, when the water in the aquarium became bad. 



The layman seeing these animals in the tubes with their 



